(fi gures 14d, 16a -d)

Description. Cell outline ovate-rhomboid, 6-11 Mm long, 4-8 Mm wide. A distinct, longitudinal dorsal keel lies to the right of the midline. Two fi ne ventral ridges may be seen towards the posterior end. The dorsal face is slightly concave between the longitudinal dorsal keel and the lateral margin of the cell. With one fl agellum about the same length as the cell inserting in a reservoir located in the right side of the cell. The nucleus is in the left side. Glides with the fl agellum directed forwards. Commonly observed. Descriptions based on observations of 28 cells.

Remarks. Larsen and Patterson (1990) fi rst described this species from marine sites in tropical Australia and Fiji. They reported lengths ranging from 7 to 9 Mm. Generally, our observations are in accordance with observations of Larsen and Patterson (1990), but the species described here has two fi ne ventral ridges not reported by Larsen and Patterson (1990). They could easily have been overlooked. The species resembles P. poosilla (see below) in having two fi ne ventral ridges and in length, but it can be distinguished by its dorsal keel and cell shape. In having one distinct dorsal keel, the species is similar to a few other species in the genus Petalomonas such as P. lata Christen, 1962, P. steinii Klebs, 1893 and P. variablilis Christen, 1962 (see Huber-Pestalozzi, 1955; Christen, 1962b). It can be recognized from all of these by its small size.